1. Field of the Invention
The present invention relates to the improvement of a control apparatus for a brush-less motor.
2. Description of the Related Art
A brush-less motor is a commonly known as rotation speed-controllable synchronous motor (which is often abbreviated to SM). In such a synchronous motor, the position of the rotor thereof must be detected in order to achieve phase control of the armature current. Under such phase control, the synchronous motor can operate with a prescribed advance control angle.
Conventionally, a mechanical position sensor (which is often abbreviated to simply PS) is coupled to the end of a rotor shaft of the motor so as to detect the rotor position, thereby achieving the phase control. Often, an electronic position sensor (often abbreviated to EPS) is used instead, which detects a counterelectromotive force of the motor to achieve phase control of the armature current.
According to a conventional brush-less motor employing an electronic position sensor, any sudden change in the load of the motor results in the phase of the counterelectromotive force of the motor varying abruptly in response thereto. When such an abrupt phase variation occurs, the advanced control angle of power conversion elements, which constitute an inverter for driving the motor, cannot be controlled precisely. As a result, the commutation margin angle of the power conversion elements decreases unduly, with the likelihood that commutation of the power conversion elements will fail. The same problem (commutation failure) will occur if the magnetic field strength of a synchronous motor or the armature current thereof varies abruptly.